The development of hemoglobin-based oxygen carriers (HBOCs) as a blood replacement bridging strategy remains an elusive but much needed objective of transfusion medicine. Complications due HBOC induced vasoactivity and a lack of understanding as to the origins of this effect as well as unanticipated differences in oxygen transport properties of HBOCs and RBC's, have hindered their development and deployment. A fully functional and highly productive consortium of well-established investigators proposes a program project composed of five projects and one cores that will: 1) Expose the underlying mechanism behind vasoactivity complications associated with the infusion of HBOCs, 2) Test new paradigms for HBOC design strategies and 3) Provide a blue print for HBOC design strategies that will allow for the control of vasoactivity and customization for therapeutic applications. The program builds upon an established, on going and highly successful collaborative efforts that have already yielded important new results including: new high yield synthetic strategies for systematic size enhancement of hemoglobins based on surface decoration with polyethylene glycol (PEG), physiology studies showing that the colligative properties of certain PEG decorated hemoglobins result in the elimination of vasoactivity, a new auto-regulatory model to account for vasoactivity and a series of biophysical results that expose functional and conformational consequences of different size enhancement and mutagenic strategies for modifying hemoglobins. The program, with Dr. Friedman as the P.I., will achieve its objectives through an orchestrated interplay among three major themes: molecule design/synthesis, molecule characterization and molecule testing. The program project consists of five projects, and a biochemical core. Project 1 (Acharya, AECOM) and 2 (Ho, Carnegie Mellon) will primarily address design strategies based on chemical and mutagenic modifications respectively. Projects 3 (Friedman, AECOM) and 4 (Peisach, AECOM) will focus on biophysical and functional characterization of surface-decorated and mutagenized HbS. Project 5 (Intaglietta, UCSD) will cover physiological testing of HBOCs. Modified hemoglobins, designed through strategies developed from Projects 1 and 2, will be produced and chemically characterized in the Protein Biochemistry Core (Dr. Manjula, AECOM) in amounts needed for Projects 2(NMR), 3(optical spectroscopy and ligand reactivity), 4(EPR) and 5(physiology).